In various fields such as power devices, wind power generators, and uninterruptible power supplies, a lithium ion capacitor including an outer container containing a lithium ion capacitor element and an electrolyte solution is recently receiving attention as an accumulator device having high energy density and high output characteristics (see, for example, Patent Literature 1). Particularly, studies on vehicle-mounting applications that require high output power are progressing, and high safety performance is strongly required by the market.
One problem particularly in a power application is that when, for example, an accident occurs, an internal short circuit may occur in a lithium ion capacitor due to deformation (folding) of the lithium ion capacitor caused by pressure from the outside or due to sticking of a sharp object into the lithium ion capacitor.
Therefore, in a lithium ion capacitor proposed to solve such a problem, a polyethylene-made plate of about 500 μm is disposed between the outer container and the lithium ion capacitor element, and the deformation due to the external pressure is thereby suppressed to improve durability (see, for example, Patent Literature 2).
However, the lithium ion capacitor configured as above has small capacitance and large resistance. Therefore, although safety can be achieved, the lithium ion capacitor does not have performance, such as high output power and high capacity, required by the market.
In a lithium ion capacitor having high energy density and high output characteristics, a large current flows when an internal short circuit occurs, causing abrupt heat generation. Therefore, there is a high possibility that the surface temperature of the lithium ion capacitor becomes high (150° C. or higher), so that, for example, a component of the lithium ion capacitor may melt. In a device (module) including a plurality of lithium ion capacitors, the influence of the heat may be exerted on a lithium ion capacitor adjacent to the lithium ion capacitor with high surface temperature, and the desired function may not be achieved.